The motive for the attacks was unclear. However, Pakistanis have viewed polio vaccination campaigns with suspicion after the CIA's use of a fake vaccination program last year to collect DNA samples from residents of Osama bin Laden's compound to verify the al Qaeda leader's presence there. Bin Laden was killed by U.S. forces in May 2011.

In June, a Taliban commander in northwest Pakistan announced a ban on polio vaccines for children in the region as long as the United States continues its campaign of drone strikes in the region, the Taliban said.

"Polio drops will be banned in North Waziristan" until the strikes cease, according to a Taliban statement. North Waziristan is a tribal region of Pakistan near the border with Afghanistan.

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Bin Laden raid hurts polio campaign

The vaccination campaign has been suspended indefinitely in Quetta, Killa Abdullah and Pishin districts, Balochistan government spokesman Noor ul Haq Baloch said. "The reason behind suspension of the campaign is the security threat to workers," Baloch said.

"At least six people working on a polio vaccination campaign have been reported shot dead in several locations in Pakistan -- Gadap, Landi, Baldia and Orangi towns of Karachi city, Sindh Province and Peshawar, Khyber Pakhtunkhwa Province," a WHO statement said. "Those killed were among thousands who work selflessly across Pakistan to eradicate polio."

WHO earlier estimated that 280,000 children living in the tribal area were in need of polio vaccinations.

"Polio is a highly infectious disease caused by a virus that can cause permanent paralysis in a matter of hours," WHO said. "Safe and effective vaccines protect children from the disease. Currently the disease remains endemic in only three countries: Afghanistan, Nigeria and Pakistan."

Health officials confirmed four cases of polio in Balochistan in 2012, while 11 cases of polio 2 virus were reported there, Baloch said. There were 73 cases of polio virus in 2011, he said.

Four female health workers were shot to death in Karachi Tuesday morning, including two women killed after administering the vaccination to children, according to Naeem Shah, a senior police official.

Two other women were shot while going house to house giving vaccinations in Peshawar, police official Javed Khan said. Gunmen opened fire on the women as they left a house where they had vaccinated children, he said.

Pakistan Prime Minister Raja Pervez Ashraf ordered his interior ministry to work with the polio eradication project to "provide foolproof security to the polio teams."